We propose to study the joining of different DNA sequences in vivo, a phenomenon that we wish to call genetic fusion in Escherichia coli K12. This phenomenon is concerned with the formation of F- and R-prime factor plasmids, of transducing phages, and of mutations such as deletions, duplications, and translocations. These studies are intended to provide new understanding of the general characteristics of genetic fusion, which is distinct from general recombination, by identifying some of the special characteristics of the DNA sequences involved. These studies will be carried out by genetic methods, by electron microscope heteroduplex mapping methods, by using enzymes, such as restriction endonucleases, S1, exo III, etc., and by DNA sequencing method developed by Maxam and Gilbert on DNA molecules or DNA fragments of F and R mutant derivatives.